Fish Belt Transect (BLT) Surveys

Fish BLT Surveys

The fish belt transect (BLT) protocol is the primary method used to quantify reef fish assemblages at UVS sites. Divers swim standardized belt transects, visually identifying and sizing all fishes observed within a defined area. This method yields robust estimates of fish abundance, biomass, and community structure across depth strata and regions (see SOP for full methods).

Fish BLT data are stored into three tables:

• uvs.blt_stations — Metadata for each station, including depth, diver, effort, and summary metrics
• uvs.blt_observations — Raw fish counts by species, size class, and transect
• uvs.blt_biomass_by_taxa — Derived biomass by station and taxon, using length-weight relationships

All records are linked via ps_station_id, and species names are standardized using taxon_id from taxonomy.fish.

uvs.blt_stations

This table contains one row per depth-stratified fish survey conducted at a UVS site. Each row includes dive metadata, survey effort (e.g., number of transects, area surveyed), and ecological summaries such as richness, total abundance, and biomass. Stations are linked to uvs.sites via ps_site_id. The ps_station_id is generated by appending a standardized depth bin label to the ps_site_id.

Standard depth strata for UVS methods

To ensure consistency across sites and protocols, station depth (depth_m) is binned into standard strata using fixed thresholds. The binned depth label is then appended to the ps_site_id to generate a unique ps_station_id.

Depth strata and labels:

• Supershallow: <= 6 m → 05m
  
• Shallow: 7–14 m → 10m
  
• Deep: >= 15 m → 20m

Examples:

• If a fish transect is conducted at 5.8 m → station ID = FJI_2025_uvs_001_05m
  
• If the same site has another transect at 14.7 m → FJI_2025_uvs_001_10m

This depth binning scheme ensures consistent stratification across analyses, regardless of slight depth variation between dives.

Table 1: Fields in the uvs.blt_stations table

Field	Type	Required	Description
ps_station_id	STRING	true	Unique station ID (ps_site_id_depth), e.g., CHL_2024_uvs_001_20m
ps_site_id	STRING	true	◂ Foreign key to UVS site table
exp_id	STRING	true	◂ Expedition ID (ISO3_YEAR)
method	STRING	true	◂ Survey method (uvs)
protocol	STRING	true	Survey protocol (fish_blt) used at the station
divers	STRING	true	Names of divers who conducted the transects (comma-separated)
date	DATE	true	◂ Survey date (YYYY-MM-DD)
time	TIME	true	◂ Survey start time (24-hour format)
depth_m	FLOAT	true	Average depth of the station (m)
depth_strata	STRING	true	Binned depth category: supershallow, shallow, or deep
region	STRING	true	◂ Region name
subregion	STRING	true	◂ Subregion name
locality	STRING	false	◂ Optional local feature (e.g., reef, bay, cove)
n_transects	INTEGER	true	Number of fish transects completed at the station
transect_length	FLOAT	true	Length of each transect (m)
transect_width	FLOAT	true	Width of each transect (m)
survey_area_m2	FLOAT	true	Total area surveyed at the station (m²)
habitat	STRING	true	◂ Habitat type (see controlled vocabulary)
exposure	STRING	true	◂ Exposure type (see controlled vocabulary)
n_species	INTEGER	false	Species richness (optional summary)
abundance_m2	FLOAT	false	Fish abundance (individuals/m², optional summary)
biomass_gm2	FLOAT	false	Fish biomass (grams/m², optional summary)
notes	STRING	false	Optional comments or QA notes

uvs.blt_observations

This table contains one row per fish observation recorded on an individual transect during a belt transect survey. Observations represent raw field data collected by divers and include species identity, estimated size, and count. Each row corresponds to a unique combination of species, size class, and transect.

• Linked to the uvs.blt_stations table via ps_station_id
• Taxonomic identity is standardized using accepted_name and aphia_id, which join to taxonomy.fish
• Fish size (length_cm) and abundance (count) are used to calculate derived metrics (e.g., biomass) in downstream summaries

Transect dimensions by fish size

• Fish ≥ 20 cm are recorded along a 25 m × 4 m belt (100 m²)
• Fish < 20 cmare recorded along a 25 m × 2 m belt (50 m²) Note: When transects deviate from standard dimensions, actual surveyed area should be recorded and used for calculations.

Table 2: fish.observations Table Schema

Field	Type	Required	Description
obs_id	STRING	true	Unique observation ID from diver datasheet (e.g., CHL_2024_blt_AMF_0001)
ps_station_id	STRING	true	◂ Foreign key linking to uvs.blt_stations
diver	STRING	true	Name of the diver who recorded the observation
station_label	STRING	false	Field-assigned depth stratum label (e.g., shallow, deep)
transect	STRING	true	Transect label: A, B, or C; OFF for observations recorded outside standard transects
depth_m	FLOAT	true	Depth at which the transect was conducted
accepted_name	STRING	true	Scientific name (Genus species) of observed taxon
aphia_id	STRING	true	Unique AphiaID from WoRMS — links to taxonomy.fish
length_cm	FLOAT	true	Estimated total length (cm) of individual or size class
terminal_phase	BOOLEAN	false	TRUE if individual was identified as terminal phase (e.g., parrotfishes)
count	INTEGER	true	Number of individuals observed in this size class
notes	STRING	false	Optional comments or QA notes

Off-transect observations

Fish observed outside of standard transects (e.g., large pelagics, megafauna) are recorded using transect = "OFF". These are included for completeness but should be treated separately in standardized analyses.

uvs.blt_biomass_by_taxa

This table contains station-level summaries of reef fish biomass and abundance by species, derived from raw BLT transect observations. It is the primary analysis-ready table for generating summaries, figures, and community structure plots.

Each row represents a unique combination of ps_station_id and species, with metrics standardized to g/m² (biomass) and ind./m² (abundance). All values are computed using species-specific length–weight parameters from the taxonomy.fish table.

Taxonomic and ecological fields — including family, trophic group, and functional group — are joined from the central species reference and enable flexible aggregation for trophic or functional analyses.

Table 3: uva.blt_biomass_by_taxa Table Schema

Field	Type	Required	Description
ps_station_id	STRING	true	◂ Foreign key to uvs.blt_stations
exp_id	STRING	true	◂ Expedition ID (ISO3_YEAR)
ps_site_id	STRING	true	◂ Site ID for spatial grouping
region	STRING	true	◂ Region name
subregion	STRING	true	◂ Subregion name
depth_strata	STRING	true	Binned depth category: supershallow, shallow, or deep
depth_m	FLOAT	true	Average depth of the station (m)
aphia_id	STRING	true	◂ Species ID (linked to taxonomy.fish)
accepted_name	STRING	false	Scientific name (Genus species)
family	STRING	false	Taxonomic family (e.g., Acanthuridae)
trophic_group	STRING	false	Ecological trophic group (e.g., piscivore, herbivore)
functional_group	STRING	false	Functional role (e.g., browser, planktivore)
count	INTEGER	false	Total number of individuals observed
abundance_m2	FLOAT	false	Average density (individuals per m²)
biomass_gm2	FLOAT	false	Average estimated biomass (g/m²)